The objective of this application is to evaluate the safety and efficacy of intravitreal injection(s) of triamcinolone acetonide in patients with macular edema. To accomplish this objective, a multicenter, randomized, controlled study comparing eyes with macular edema that receive intravitreal injection(s) of triamcinolone acetonide to eyes with macular edema that receive conventional therapy is proposed. Patient Population (inclusion criteria): -Macular edema secondary to diabetic retinopathy, central retinal vein occlusion (CRVO) or branch retinal vein occlusion (BRVO). -Macular edema must involve the center of the fovea and area of macular edema must be equal to or greater than 0.44 disc area (area of central subfield of the ETDRS grid). -Eyes must have received at least one prior session of grid/focal photocoagulation prior to study entry (diabetic retinopathy and BRVO subgroups) and macular edema must have persisted for at least 90 days following treatment. Eyes with CRVO may be enrolled if macular edema persists for 90 days or more. -Visual acuity score of > 19 letters (20/400) and < 68 letters (20/50) by ETDRS visual acuity protocol in the study eye. Treatment vs. Control Groups: Eyes randomized to the treatment group will receive an intravitreal injection of triamcinolone acetonide (4mg/0.1 cc). Eyes randomized to the control group will be observed or treated with repeat grid/focal photocoagulation in the diabetic macular edema and BRVO subgroups and observed in the CRVO subgroup. Efficacy and Safety Assessments: The primary efficacy endpoint will be improvement by 10 or more letters from baseline in best-corrected visual acuity as determined by the ETDRS visual acuity protocol. Secondary efficacy outcomes will include area of retinal thickening and retinal thickness as assessed by stereoscopic fundus photography and optical coherence tomography. Potential adverse events such as endophthalmitis, retinal detachment, vitreous hemorrhage, cataract and glaucoma will be monitored with respect to frequency and severity.